The present invention provides a novel method and system for reducing the sulfur (sulfates or sulfites) content of a wastewater stream to be treated anaerobically. The reduction of sulfur content in wastewater according to the present invention provides for feeding of wastewater high in sulfur content to a reactor and maintaining the reactor at a pH level less than 6, wherein the sulfate and/or sulfite compounds in the wastewater are converted into sulfide gas and exhausted from the reactor.
Two-phase anaerobic processes are well known as demonstrated in U.S. Pat. Nos. 4,067,801, 4,318,993 and 4,022,665. The anaerobic process is the use of a biological process for the stabilization of organic material by conversion to methane gas in the absence of oxygen. The process normally consist of two groups of bacteria, acid-forming bacteria which convert organics to volatile acids and finally to acetic acids and H.sub.2, and a second group of methane-forming bacteria which convert acetic acids, H.sub.2 and CO.sub.2 to methane gas. Although both acid-forming bacteria and methane-forming bacteria may operate concurrently in a single reactor it is preferable, as described in the aforementioned patents, to have separate reactors for acid fermentation and methane fermentation.
This two-phase anaerobic process for treating wastewater is satisfactory in most instances. However, in instances where the wastewater is high in sulfate (SO.sub.4.sup.=) and sulfite (SO.sub.3.sup.=) content, such as wastewater from pulp and paper processing, the sulfates and sulfites are reduced to sulfides under conditions of an anaerobic reactor. Sulfides, such as H.sub.2 S and HS.sup.-, are known to be toxic to methane-forming bacteria at a level as low as 150 mg/liter, and as such many sulfate and/or sulfite containing wastewaters cannot be treated or disposed of anaerobically.
If sulfates and sulfites are present in a wastewater at a concentration above 150 mg/liter then they may be toxic to methane-forming bacteria, thus preventing or slowing the conversion of acetic acid, H.sub.2 and CO.sub.2 to methane gas. This effectively prevents the use of anaerobic systems in the treatment of sulfur containing wastewaters. The addition of metal salts, such as ferrous or ferric chloride, to the reactor to precipitate out of the metal sulfides has been used to reduce the sulfide concentration to allow anaerobic treatment of wastewaters high in sulfur content as described in U.S. Pat. No. 4,318,993. However, the effect of ferrous ions in the anaerobic process, and the cost of chemicals required, have reduced the effectiveness of the anaerobic process for handling these types of wastewaters.
Thus, the present inventor has discovered a method and system for sulfide toxicity reduction in the anaerobic treatment of wastewater high in sulfur content which is both economical and satisfactory in anaerobically treating such wastewaters. The present invention overcomes the problems and expense involved in reducing the sulfide contents in wastewater by means of addition of metal salts or chemicals such as ferric or ferrous chloride to precipitate out metal sulfites and removal of sulfide precipitates from the reactor. Additional advantages of the present invention shall become apparent as described below.